Typical transmission systems such as modern CATV (community access television) systems include a multitude of individual pieces of equipment to generate, amplify, split, combine and distribute the carrier multiplexed signal to end users. There are 2 main types of carrier multiplexed signals that are typically distributed, legacy analog and complex digitally modulated QAM (quadrature amplitude modulation) signals. These can further be divided into 2 categories, broadcast and narrowcast in which broadcast signals are sent to many end users while narrowcast signals are sent to a limited subset of end-users. Typically, legacy analog signals are broadcast while QAM modulated signals can be broadcast or narrowcast usually depending upon content type.
There are 4 main types of content that can be sent to the end user using QAM: continuous streaming video, switched digital video (SDV), video on demand (VOD) and data over cable such as internet and phone (DOCSIS {data over cable service interface specification}). Continuous streaming video is generally broadcast while SDV, VOD and DOCSIS content is generally narrowcast. Each content source generally comes from separate pieces of equipment with QAM outputs that must be combined in the RF (radio frequency) domain, with legacy analog if desired, to create the broadcast and narrowcast channel lineups that are distributed by the HFC (hybrid fiber coax) distribution equipment.
As the demand for narrowcast increases, the narrowcast service groups are generally segmented into a smaller and smaller number of end users. However, every time a service group is divided in half, the amount of equipment needed for narrowcast services effectively doubles. Because of this, space quickly becomes a limiting issue in many head-ends. In addition, channel line-up reconfiguration, power consumption and physical RF cable management are also becoming major issues.
To overcome these issues, there has been a push in the industry for higher density and convergence of the many disparate pieces of equipment needed to generate and distribute the various types of signals into a more unified system that will save space, reduce the amount of RF cabling required within the head-end, make network management and channel line-up reconfiguration easier and reduce power consumption.
To this end, a standard, interchangeable, small form factor analog pluggable optical module has been proposed. Such a module could be plugged into a high density HFC transmission system or a unified platform that generates the modulation, combines the channel line-up and drives the pluggable optical module directly. The output of such a module would connect directly into the existing fiber-optic distribution network. This would allow for increased density, more unification and modular serviceability.
However, in order to obtain similar noise and distortion performance as what is obtained in traditional HFC distribution equipment, predistortion of the signal that modulates the laser in the pluggable optical module may be required. Traditional HFC equipment often predistorts the signal that drives the laser modulation in such a manner that it compensates for the distortion produced in modulation and transmission of the signal. This helps make the received signal more linear which reduces noise and distortion.
Fitting some or all of the desired predistortion circuitry into an analog small form factor pluggable optical module may not be possible due to the limited space. It may also not be cost effective or work as effectively as desired. There may also be cost considerations that prevent inclusion of the predistortion circuitry in the small form factor pluggable optical module.